Various types of synthetic resin film packing bags are widely used for the packaging of food, pharmaceutical compositions, sundry goods and the like. They are produced mostly by laminating a biaxially oriented synthetic resin film and a heat-sealable non-oriented film in two or more layers. Especially, biaxially oriented high strength polyamide films produced by means of flat simultaneous biaxial drawing, flat serial biaxial drawing, tubular method and the like are used as the biaxially oriented films in the field where durability, heat resistance, oil resistance, solvent resistance, mechanical strength and the like are required.
Since these packing bags in which biaxially oriented polyamide films are used have a problem of poor unsealabilility by tearing, they are sometimes equipped with notches as a means to improve their unsealability. However, when these bags are torn from the notch, frequently they cannot be torn in a lengthwise or crosswise direction easily or linearly. In such a case, unsealing occurs along a distorted line and excess force is required due to forced tearing, so that not only are the contents scattered and wasted but also accidents such as soiling of clothes occur when the contents are liquids. In order to resolve such problems, a method is employed in which a bag having reduced sealing strength is produced by laminating a biaxially oriented polyamide film and a heat-sealable non-oriented film, and the bag is unsealed by tearing it from the part sealing the bag. However, this method still requires further improvement, because, in some cases, unsealing of the bag is not easy and its contents scatter from the sealing part during transfer handling of the sealed bag.
Laminated products in which a uniaxially oriented polyolefin film is used as an intermediate layer, such as a three layer laminated film of biaxially oriented polyamide film/uniaxially oriented polyolefin film/non-oriented polyolefin film, are known as easily unsealable materials which show excellent tear linearity when films are torn. In this case, tear linearity in the drawn direction of the uniaxially oriented film is improved, but it is necessary to construct a multilayer structure by newly inserting an intermediate layer for the purpose of merely improving the tear linearity, thus resulting in high cost, limited use and like problems.
With regard to easily tearable films, a biaxially oriented film of mixed polyamide prepared by mixing N6 with 15 to 60% by weight of MXD6 (polymetaxylylene adipamide) and laminated films thereof have been proposed in JP-A-5-220837 and JP-A-5-200958 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, such MXD6-mixed films have a problem in terms of pinhole resistance, particularly frequent generation of pinholes due to bending fatigue.